The present invention relates to a shadow mask frame assembly for a CRT, and more particularly to an improved hook spring for fixedly installing the shadow mask frame assembly to a stud pin on the inner side of a panel.
Generally, as shown in FIG. 1, a CRT comprises a shadow mask 6 for performing a color selection function to land an electron beam 2 projected from an electron gun 1 onto a phosphor surface 5 formed on the inner surface of a panel 3. Shadow mask 6 with countless electron beam-passing holes 7a has a beam-passing plane 7 perpendicular to a proceeding axis X--X of the electron beam and is welded to flames 9 by skirts 8 formed on the periphery of the shadow mask.
As shown in FIG. 2, a plurality of hook springs 10 having a fixing hole are fixed to the outer surface of frame 9 perpendicular to the electron beam proceeding axis X--X and in the same rotational direction. Respective stud pins 4 formed on the inner surface of panel 3 are fitted into the fixing holes of hook springs 10 so that shadow mask 6 is mounted onto panel 3, staying a certain distance apart from the inner surface of panel 3.
However, only 15-18% of electron beam 2 projected from electron gun 1 passes through electron beton-passing holes 7a of shadow mask 6, and the remainder collides against shadow mask 6. The colliding rays of electron beam 2 heat shadow mask 6 which is thermally expanded to have a doming phenomenon wherein the center portion of shadow mask 6 swells. Due to the doming, the periphery of shadow mask 6 has a greater discordance between the proceeding axis X--X of electron beam and electron beam passing holes 7a so that, in effect, electron beam 2 does not precisely land on phosphor surface 5, causing the deterioration of the picture's colorimetric purity. Such a problem can be solved by moving shadow mask 6 slightly toward phosphor surface 5 so as to accord electron beam-passing holes 7a with the proceeding axis X--X.
In FIGS. 3A, 3B and 3C, a conventional hook spring 10 for compensating for the shadow mask's position is illustrated which suppresses the reduction of colorimetric purity. One end of hook spring 10 is fixedly welded to frame 9 via a bimetal member 12 welded on the frame's outer side. A fixing hole 11 for fitting stud pin 4 of panel 3 is formed on the free end of hook spring 10. The bimetal member 12 is affixed to frame 9 so that, during thermal expansion of shadow mask 6 and frame 9, shadow mask 6 moves toward phosphor surface 5 of panel 3. In some cases, in order to cope with thermal expansion, the hook spring 10 is made of a plate-like material called parallel stick-together (PST) which is a stack of metals having different thermal expansion coefficients.
When the temperature of bimetal member 12 goes up, the conventional hook spring 10 expands most in an unwelded portion so that the hook spring is distorted in the direction of the electron beam's proceeding axis as well as perpendicular thereto, by certain angles .THETA.1 and .THETA.2. Thus, the shadow mask frame assembly is moved by a certain distance toward the phosphor surface to compensate for the landing position of the electron beam with respect to the thermal deformation of shadow mask 6.
However, since the movement of the conventional shadow mask frame assembly is determined by the deformation angle .THETA.1 of hook spring 10 in the direction the beam's proceeding axis and by the deformation angle .THETA.2 of hook spring 10 in the direction perpendicular to the beam's proceeding axis, the shadow mask frame assembly becomes abnormally rotated or distorted. This hinders the sufficient compensation for the position of shadow mask 6 with respect to its thermal expansion. Further, the conventional shadow mask frame lo assembly has a complicated manufacturing process in which a bimetal member 12 is welded onto locations around frame 9 and hook spring 10 is welded to each bimetal member 12. Besides, since the shadow mask frame assembly is supported by hook spring 10 extended from bimetal member 12 by a certain distance, the assembly is easily shaken by minor external impacts.